1. Field of the Invention
The present invention relates to a boosting circuit, and more particularly, to a boosting circuit to be used for a high frequency equalizer without having the amount of boost affected by external factors of not only frequency operation characteristics, but also voltages, processes, temperatures, and so on.
2. Description of the Related Art
The boosting circuit for an equalizer employed in optical recording/reproducing devices or hard disk drives for compact disk (CD)/digital video disk (DVD) is required to output differential currents from input voltages, and to adjust the amount of boost without affecting group delay characteristics up to a high frequency bandwidth of about 100 MHz.
FIG. 1 is a view for illustrating a conventional boosting circuit. As shown in FIG. 1, the conventional boosting circuit is constructed with capacitors C1 and C2, resistors R1 and R2, an operational amplifier A1, and a transconductor Gm1. The operational amplifier A1 is a fully differential operational amplifier having two input terminals and two output terminals.
In such a circuit constructed as above, an input voltage Vi is differentiated in a differentiation circuit constructed with the operational amplifier A1, capacitors C1 and C2, resistors R1 and R2 and converted into a current for output through the transconductor Gm1. If R=R1=R2 and C=C1=C2, the relationship between the input voltage Vi and an output current io(=iop−ion) can be expressed in Equation 1 as follows:io(s)=gmRCsvl(s),  [Equation 1]wherein gm denotes a transconductance of the transconductor Gm1. Accordingly, the magnitude of output current io can be adjusted by varying the transconductance gm of the transconductor Gm1.
However, such a boosting circuit is affected by external factors such as temperatures, voltages, processes, and so on, since the magnitude of output current io is adjusted by varying the transconductance gm. Therefore, there is a drawback in that an additional tuning circuit is needed to prevent such influences. Further, there exists a problem in that power consumption becomes high and frequency characteristics are deteriorated, since a two-stage process is needed to differentiate and then convert input voltages into currents.
FIG. 2 is a view for showing another conventional boosting circuit. The boosting circuit of FIG. 2 has capacitors C3 and C4, an operational amplifier A2, and NMOS transistors M1, M2, M3, M4, M5, and M6.
In such a circuit constructed as above, an input voltage Vi is converted into a differential current through the operational amplifier A2 and the capacitors C3 and C4. If C=C3=C4, the relationship between the input voltage Vi and an output current io(=iop−ion) can be expressed in Equation 2 as follows:io(s)=kCsvl(s),  [Equation 2]wherein k denotes a size ratio, that is, Width(W)/Length(L), of NMOS transistors M1 and M2 or M4 and M5. Therefore, an amount of output current can be adjusted by using a size ratio of NMOS transistors.
In the boosting circuit of FIG. 2, an extra tuning circuit is not needed since the amount of output current is adjusted with the size ratio, k, of the MOS transistors, resulting in an advantage of directly converting an input voltage into a differential output current, compared to the boosting circuit of FIG. 1.
However, unlike the boosting circuit of FIG. 1, an output of an output amplifier constructed with the NMOS transistors M1 and M4, current sources IB, and so on, is used as an input to the NMOS transistors, so that the input terminals of the operational amplifier A2 are formed with p-type elements, which causes a drawback in broadening bandwidth since the p-type elements have a small transconductance compared to the n-type elements. Further, the two-stage amplification structure formed with the operational amplifier A2, NMOS transistors M1 and M4, and current sources IB results in a drawback in that frequency compensation is needed for stability. Moreover, the DC bias currents IB to the NMOS transistors M1 and M4 are reflected in the NMOS transistors M2 and M5, which, as a drawback, require compensations at a bias stage connected to a rear stage of the differentiation circuit.